A. Field of the Invention
The present invention relates generally to interferometric devices and more particularly to quadrature fiber-optic interferometers.
B. Description of the Prior Art
Homodyne systems are commonly used in the field of optical interferometers for measuring an environmental parameter such as pressure, temperature, force, magnetic fields, electric fields, acoustic waves, etc. A homodyne system is one in which light carried in both a signal source fiber and a reference fiber are of the same frequency. The signal source fiber is modulated by an environmental parameter to change the phase of the light traveling in the signal source fiber with respect to the reference fiber. Subsequent coupling of the signal and reference fibers produces interference between the two signals which results in a fringe pattern related to the applied physical effect. Maximum sensitivity of a homodyne interferometer is achieved when the path difference between the reference and signal fibers produced by the applied physical effect is an integral odd multiple of .lambda./4 where .lambda. is the wavelength of the light in the fiber. At these points, the interferometer has maximum sensitivity to small differential phase shifts. On the other hand, when the pathlength difference is an integral multiple of .lambda./2, the interferometer is least sensitive to small differential phase shifts.
One problem with the homodyne system is that the phase difference tends to drift slowly under operating conditions due to temperature changes, pressure changes, etc. Thus, the sensitivity of the interferometer is not constant for long periods of time unless a means of actively controlling, or tracking, the phase shift is provided. This is commonly done with a piezoelectric transducer to make minor adjustments in the phase difference between light in the reference fiber and the signal source fiber. While this method has been useful in the past, some disadvantages to this kind of dynamic tracking homodyne system are that it does require additional electrical circuitry which may not be desirable in some environments, and it does require additional adjustments during operation.